Robertshaw Controls Co., assignee of the present invention, sells a liquid level sensor for detecting the presence or absence of a liquid and providing an output that activates an alarm. This prior art sensor is sold under the designation Mini-Tek 800. The Mini-Tek 800 sensor has a probe that determines a liquid level at a probe site by monitoring capacitance changes caused by changes in liquid level.
U.S. Pat. No. 4,224,606 discloses a fluid level control circuit having a test function. Pressing a test switch reduces voltage at an FET gate and takes the FET from a pinched-off condition allowing a drain capacitor to discharge through the FET. This drops a relay out of the circuit and closes an alarm contact to actuate an alarm.
U.S. Pat. No. 4,676,100 discloses a test circuit for simulating a "high" material level in an LC resonant frequency fluid level indicator. Closing a switch activates a second switch and adds three capacitors in parallel with the probe. The increased capacitance causes a threshold detector to indicate a "high" material level.
U.S. Pat. No. 5,121,632 discloses a sensor including a self-diagnostic routine which connects the probe to a comparator that compares the voltage on the probe to a supply voltage. Under normal conditions, the variable capacitance of the probe will charge up to a supply voltage. If the probe is shorted, then the variable capacitor will not charge up and cause the output of the comparator to be high to flag and error condition on a microprocessor controller.
The present invention concerns a test circuit for use in conjunction with a sensor such as the prior art Mini-Tek 800. A test input allows an operator to artificially simulate a condition which normally would cause the circuit to activate an alarm. By activating the test input, the circuit's operation can be confirmed.